What would be a good thesis for an essay on Oedipus Rex?

A good thesis must state an opinion, meaning it must be arguable, and it must be specific. For instance, saying "Oedipus Rex is about pride or hubris" is not a good thesis, first, because nobody would argue about that, and second, because is it too general.

However, pride is a good place to start in writing an essay about this play. Oedipus is a very proud man. One way to argue would be to say something like the following: "On the whole, Oedipus's pride works for the common good but also leads to his personal downfall because he is blind to his own shortcomings."

You would then want to find support for your thesis. You could talk about how Oedipus's pride in caring for the well-being of his people works to their advantage: even before they come to petition him for relief from the plague, he has been proactive, sending Creon to get advice from the oracle at Delphi. Oedipus, also, in his opening speeches, expresses his compassion for his people. His pride also keeps him from backing down when he is warned that he should not keep investigating what led to the plague: he is confident he can get to the bottom of it, and that pride in his abilities is what the people need.

Where pride goes wrong is in Oedipus's blindness to the possibility he could be the cause of the plague. If he had listened to the warnings as a hint of his own wrongdoing, he would have been prepared. But he thinks he can do no wrong, and the shock that he can almost destroys him. You could talk about how his having the self-awareness that he might be in the wrong might have allowed him to prepare Jocasta so that she didn't kill herself, and allowed him to prepare so that he wouldn't stab his eyes out. You might conclude that what was good for the people, ending the plague, was bad for him personally.

Can you help me develop a thesis about Oedipus the King?

Given that Oedipus' character (from Sophocles' play "Oedipus Rex") was critically examined by Aristotle in his text Poetics, one could make an argument that Oedipus does not fit perfectly into the definition of Shakespeare's typical tragic hero.

While Oedipus does possess a tragic flaw (or hamartia as defined by Aristotle), one could argue that Oedipus does not hold up, characteristically, to what is expected of a modern true tragic hero.

To do this, one would need to look at the characteristics of both Aristotle's and Shakespeare's tragic hero.

According to Aristotle, a tragic hero must possess the following ideals/characteristics: noble statue and possess greatness, occupy a high position and embody virtue, be considered great (but not perfect), the downfall is the fault of the hero, misfortune is not completely brought about by their own actions, their fall increases self-discovery, the hero's fall does not leave the audience (or reader) depressed.

According to Shakespeare, a tragic hero must possess the following ideals/characteristics: downfall due to pride, doomed from the start, typically a leader (or king), suffering must be for a reason, tragic hero is typically male.

In order to prove your thesis, examine and support the textual evidence which proves Oedipus to not adhere to the tragic hero characteristics.

Another thesis you could develop is how the environment which surrounded Oedipus had more influence over his actions than his own decisions. Basically, you would be looking at the concept of nature verses nurture.

Can you help me develop a thesis about Oedipus the King?

The thesis, like you said, must be arguable.  Avoid the common mistake of writing a thesis that simply states a fact.  Consider Oedipus' character trait that leads to his downfall, his attempt to change his fate, the irony in his symbolic and physical blindness throughout the play, or the lesson he learns about himself and his life in the end.  You could build on one of those ideas or even combine a couple of them into an arguable thesis.  Just remember to go with an argument that you can support with strong examples from the play.